DE102009031773A1 - Potentiometric sensor for the combined determination of the concentration of a first and a second gas component of a gas sample, in particular for the combined determination of CO2 and O2, corresponding determination method and use thereof - Google Patents

Abstract

The present invention relates to a potentiometric sensor for the combined determination of the concentration of a first and a second gas constituent of a gas sample (P) which can be introduced into or present in a measurement gas space (1)
a cation-conducting, in particular a group-1-cation-conducting, in particular a Na + -ion-conducting, solid electrolyte (2),
a measuring electrode (3) having a first gas-sensitive material with respect to the first and / or the second gas constituent, which can be brought into gas contact with the measuring gas space and / or standing and which is in surface contact with the solid electrolyte;
a reference electrode (4) having a second gas-sensitive material with respect to the first and / or the second gas constituent, which can be brought into gas communication with the measurement gas space and / or standing in contact with the solid electrolyte and electrically isolated from the measurement electrode;
a reference electrode (5) which is in surface contact with the solid electrolyte (1) and is electrically isolated from the measuring electrode and the reference electrode and gas-tight against the measuring gas space and / or closed.

Description

The present invention relates to a potentiometric sensor for combined determination of the concentration of a first and a second gas component of a gas sample. The first and the second gas component of the gas sample may be, in particular, carbon dioxide CO ₂ and oxygen O ₂ . The invention also relates to a corresponding determination method and to the use of such potentiometric sensors and such determination methods.

The combined measurement of CO ₂ and O ₂ concentration in gases is of great interest in many applications. For example, in indoor air monitoring (eg in offices or in the interior of a vehicle), the CO ₂ concentration plays a decisive role in the well-being and concentration of the occupants and is therefore a decisive parameter, for example. B. for the regulation of automated air conditioning. In contrast, in rooms in which, for example, operated with nitrogen cooled devices, the oxygen concentration monitored for safety reasons.

Another important area of application is respiratory gas analysis, which measures CO ₂ and O ₂ concentrations in respiratory gas during inhalation and exhalation, and may be used to determine personal endurance performance, taking into account other data.

In the recent past, carbon dioxide as a greenhouse gas has become more and more of a public interest due to global warming and increasing levels of atmospheric CO ₂ . In response, stricter emission limits and more detailed monitoring and control of exhaust gases from combustion processes have been adopted in many countries. Here, too, measuring systems for determining the CO ₂ and O ₂ concentrations play an important role.

In principle, the CO ₂ and O ₂ concentrations as parameters in many other industrial, medical and / or biological applications represent an important measure. This results in a steady demand for cost-effective robust and long-term stable sensors for the measurement of O ₂ concentration and the CO ₂ concentration.

While various reliable and cost-effective electrochemical measurement methods (for example the lambda probe o., Limit current probes) based on oxygen ion conductive solid electrolytes such as. B. established with yttria stabilized zirconia, reliable and, above all, cost-effective CO ₂ sensors are still the subject of current research and development. Various sensor principles are known with which the CO ₂ concentration in gases can be determined. Some examples of this are optical sensors and electrochemical sensors, which can be further classified in z. As amperometric sensors, semiconductor sensors, potentiometric sensors and some other sensor types. Potentiometric CO ₂ sensors have crucial advantages, since they are very inexpensive through high-throughput techniques such. As the screen printing technology can be produced to provide an electrical measurement signal that is proportional to the logarithm of the CO ₂ partial pressure and thus allow a very simple data processing, and allow a good miniaturization of the sensors resulting from the measurement principle.

In order to allow a measurement of both the CO ₂ , and the O ₂ concentration in gases, nowadays separate oxygen and carbon dioxide sensors are usually used together. The sensors, which are generally based on different measuring principles, generally require two separate evaluation devices which increase the complexity of the measuring system and thus increase manufacturing and operating costs. So z. For example, the use of two separate potentiometric sensors for measuring either the CO ₂ or the O ₂ concentration is known. Also, the measurement of the CO ₂ or O ₂ concentration is known by two separate sensors based on different measurement principles (amperometric and potentiometric).

The object of the present invention is thus to provide a potentiometric sensor which enables a combined determination of the concentration of a first and a second gas component of a gas sample (in particular: the CO ₂ content and the O ₂ content in the gas sample). In the context of the present invention, a combined determination is understood to be a determination of the two gas fractions in which both gas fractions are not determined with individual sensors but with a single integrated sensor unit which is designed to determine the concentration of both the first and the second gas constituent , (This means that the sensor unit can separately determine each of these two concentration values, not, for example, only the sum of both concentrations can determine.)

task moreover, the present invention is a to provide such a potentiometric sensor, which allows a high degree of miniaturization, requires a minimum of complexity of the measuring system and enables a simple and cost-efficient production.

moreover It is the object of the present invention, such a sensor to provide a combined measurement the two gas concentrations at the lowest possible operating temperatures (≤ 600 ° C) allows to minimize operating costs and / or one to allow mobile application.

Also It is the object of the present invention to provide such a sensor to provide a self-diagnosis or a simple calibration routine of the measuring system allowed and a high long-term stability of the measuring signal over several Months to years.

After all It is the object of the present invention, a corresponding Determination method for the combined determination of two concentrations of gas components.

These Task is achieved by a potentiometric sensor according to claim 1 and by a corresponding determination method according to claim 21 solved. Advantageous embodiments of the inventive sensor and the invention Determination method can each be dependent Remove claims. invention Uses are described in claim 23.

following The present invention will first be general, then described in detail with reference to several embodiments. The in the embodiments in combination with each other realized advantageous features of the present invention can be within the scope of the present invention (their scope described by the independent claims is) independently of each other and / or in other combinations realize each other. In the context of the present invention must Therefore, not all individual features used in the embodiments are shown to be realized in combination with each other, but It may also be omitted individual advantageous features or individual features in other combinations with each other be realized.

In the context of the following description, a flat contacting of a first element with a second element means that the two elements touch, ie directly adjoin one another, wherein the area in which the two elements touch flatly (eg with a size in the square millimeter range preferably between about 0.4 mm ² and 30 mm ² ) is formed and not z. B. only punctiform. If, in the context of the invention, a measurement gas space and / or a reference gas space are used, these may be both gas spaces which are part of the described sensor according to the invention and gas spaces which are not part of the sensor but which are introduced by introduction of the sensor in the corresponding space are opened by the sensor (in the latter case by areas in which the sensor or at least a portion of a surface thereof can be introduced or adjacent to which the sensor can be arranged or surface areas thereof can be arranged ).

Of Further will be in the context of the following description under a Printing a functional element a mechanical printing process understood this from one of the five main printing processes: high, Low-, low- and high pressure or the instant printing process (non-impact) consists. Examples of such processes are screen printing (Printing) the pad printing (gravure printing) or the inkjet printing also Ink-Jet (instant print) dar. Here are the functional Components of a layer, for example in the form of a paste, an ink or a slip. The functional components such as metals and / or metal salts are usually considered as finely divided powder in a mixture of organic binder and / or Solvent. The additional components such as solvents and binders can after printing be removed by a heat treatment so only the functional components remain as a layer. By a suitable heat treatment may have the characteristics such as mechanical stability or electrochemical Activity of the layers influenced or adjusted.

Other technologies for making and contacting functional elements include molded articles which may be formed from compressed components of the functional components. Such elements can then be brought into contact with each other and, for example, connected to one another via printed layers and a heat treatment. Also spraying in which the functional Components are applied directly or in a mixture of organic binder and / or solvent provide a way to produce individual elements and bring them into contact. Here, too, desired properties of the functional elements can be achieved by a heat treatment.

The basic principle of the present invention is based on a galvanic cell with cation-conducting solid electrolyte and with two solid-electrolyte contacting electrodes of different solid-state mixtures. The basic principle of such a sensor technology is described in the literature with reference to a CO ₂ sensor and is known to the person skilled in the art (see, for example, US Pat. Sensors and Actuators, 12 (1987) W. Wepner - SOLID-STATE ELECTROCHEMICAL GAS SENSORS *, pages 107-119 and Sensors and Actuators, 12 (1987) G. Hotzel and W. Wepner - POTENTIOMETRIC GAS SENSORS BASED ON FAST SOLID ELECTROLYTES *, pages 449-453 ).

A potentiometric sensor according to the invention for the combined measurement of the concentration of two gas constituents has a cation-conducting (eg Na ⁺ ion conducting) solid electrolyte, one in gas communication with a sample gas space (containing the gas sample with the two gas constituents) and with the solid electrolyte measuring electrode in area contact (which is gas sensitive with respect to the first and / or second gas constituents), a reference electrode in gas contact with said measuring gas space in physical contact with the solid electrolyte and electrically insulated from the measuring electrode (which or the second gas component is gas-sensitive) and a solid contact with the solid electrolyte, from the measuring electrode and from the reference electrode electrically isolated and gas-tight relative to the measuring gas chamber closed reference electrode.

Prefers the sensor is designed so that the potential difference between the measuring electrode and the reference electrode, the potential difference between the measuring electrode and the reference electrode and the potential difference determinable between the reference electrode and the reference electrode and are evaluable. The measurable difference of the individual electrical Potentials d. H. the voltage between the respective electrodes involved one of the aforementioned pairs of electrodes (also known as electromotive Force (EMF) refers directly to, for example. Via a voltmeter be measured. The determination of the concentration of the first and of the second gas component from the measured voltages will be hereinafter still described in detail.

Of the Sensor according to the invention is advantageous designed so that the voltage between the measuring electrode and reference electrode only from the partial pressure of the first gas component (not from the partial pressure of the second gas component), that the voltage between reference electrode and reference electrode only the partial pressure of the second gas component (not depends on the partial pressure of the first gas component) and that the voltage between the measuring electrode and the reference electrode both from the partial pressure of the first and the partial pressure of the second Gas component depends.

This can be realized as follows:
The individual electrodes may be formed as electrode grid, for example in the form of printed, partially transparent layers; the individual electrodes may have electrode leads in the form of metal wires and / or printed layers. The electrodes consist (except for the reference electrode) of a specific for the gas type to be analyzed material or a mixture of materials hereinafter referred to as gas-sensitive material and is characterized in that it is a redox reaction such as. In Eq. (1), cf. below, with at least one of the gas constituents to be determined may have. By suitable selection of the gas-sensitive material on one of the electrodes in contact with the measuring gas chamber, as well as the additional introduction of such electrodes concentrations of various gas components can be determined. An example of this is, in addition to the determination of O ₂ and CO _2, the determination of the NO ₂ concentration by, for example, the choice of NaNO ₃ as the gas-sensitive material (see above reference): In this way, according to the invention, a combined determination of O ₂ and NO ₂ as well as a combined determination of O ₂ and CO ₂ .

Particularly preferably, the sensor is designed such that CO ₂ can be determined as the first gas constituent and O ₂ as the second gas constituent. By means of the sensor according to the invention, however, other gas component pairs (for example NO ₂ , CO ₂ and O ₂ ) can also be determined with a suitable choice of material for the gas-sensitive electrodes.

The gas-tight termination of the reference electrode with respect to the sample gas space can by means of a ga simpermeable layer be realized. This layer may be a sintered glass layer; However, this gas-impermeable layer can also be formed from the same material as the solid electrolyte.

In In another variant, the reference electrode of the present Invention also of a gas-sensitive material (gas-sensitive with respect to the first and / or the second gas component) be formed and with the solid electrolyte in planar Standing in contact. In this case the sensor works with one of the Gas chamber gas-tight separated reference gas space, wherein the reference electrode then in gas communication with the reference gas space (but not with the sample gas space) is. Sample gas space and reference gas space are on one and the same solid electrolyte formed and can thereby formed on two opposite sides thereof be, wherein the gas-tight separation of these two gas spaces preferably by means of the solid electrolyte itself and / or by means of a separate gas seal, for example by means of a be formed with the solid electrolyte gas seal can. This seal can be a glass seal. measuring electrode and reference electrode can thereby on a surface be formed of the solid electrolyte, the reference electrode on the opposite surface of the solid electrolyte.

The in the context of the invention usable materials for the Reference electrode, the measuring electrode and the reference electrode as well for the solid electrolyte will be described in detail below.

Around to ensure an electrical separation of the individual electrodes can be connected between two electrodes (eg the measuring electrode and the reference electrode) an inert barrier layer (eg made of glass) is arranged be, the short-circuit currents on the surface of the solid electrolyte between the two corresponding electrodes prevented. This barrier layer may be in an advantageous embodiment also by the gas-impermeable covering layer of the reference electrode be realized what in terms of miniaturization of the invention Sensor is beneficial. An electrical isolation between two However, electrodes can also be separated by a sufficient distance Both electrodes are ensured from each other.

The fundamental principle of the potentiometric sensor according to the invention will now be described on the basis of a determination of the CO ₂ concentration and the O ₂ concentration in a gas sample on the basis of FIG 1 to 6 described. This is followed by two concrete examples for the implementation of this basic principle ( 7 and 8th ). By appropriate choice of material, this principle can be readily transferred within the scope of the invention to the determination of other pairs of gas constituents (eg SO ₂ / O ₂ or NO ₂ / O ₂ ).

The exemplary combined potentiometric CO ₂ -O ₂ sensor principle, which is characterized by a simple structure (only one cation and in particular a sodium ion-conducting solid electrolyte layer), can be designed so that all the individual components are produced entirely by screen printing. The understanding of the basic mode of operation of a sensor according to the invention will be described below by way of example with reference to a potentiometric CO ₂ sensor ( 1 ).

• (a) tragendes Substrat bspw. aus einer Al₂O₃ Keramik.
• (b) Natriumionen leitende Festelektrolytschicht bspw. aus Nasicon oder Na-β/β''-Al₂O₃,
• (c) Elektrodenkontaktgitter und Ableitungen (aus Gold oder Platin) zum Messgerät,
• (d) inerte Barriereschicht aus bspw. einem Glas, durch welche die Kurzschlussströme auf der Oberfläche des Festelektrolyten und ein Elektrodenkontakt auf Grund von Material „ausfließen” aus den Elektroden verhindert werden soll.

The following applies to the individual components:
ME measuring electrode, for example from a mixture of Na ₂ CO ₃ and Au (or Pt),
RE reference electrode, for example. Of a mixture of Na ₂ Si ₂ O _5, SiO ₂ and Au (or Pt),

• (A) supporting substrate, for example. From an Al ₂ O ₃ ceramic.
• (b) sodium ion-conducting solid electrolyte layer, for example, from Nasicon or Na-β / β "-Al ₂ O ₃ ,
• (c) electrode contact grid and leads (gold or platinum) to the meter,
• (d) inert barrier layer of, for example, a glass, through which the short-circuit currents on the surface of the solid electrolyte and an electrode contact due to material "flow out" is to be prevented from the electrodes.

The electrochemical equilibrium reactions or redox reactions of the gas-sensitive electrode materials with the gases CO ₂ and O ₂ in the measuring gas are reproduced at the two electrodes of the sensor: ## EQU2 ## in equation (1) and (2) At the ME: Na ₂ CO ₃ ⥨ 2Na ⁺ + 2e ^- + ½O ₂ + CO ₂ (1) At the RE: 2SiO ₂ + 2Na ⁺ + 2e ^- + ½O ₂ ⥨ Na ₂ Si ₂ O ₅ (2)

The, as in 1 admixtures of gold or platinum, usually in the form of a finely divided powder which forms electrically conductive paths in the electrode, acts on the one hand in the two reactions as an electrochemical catalyst by the discharge of the free-moving (delocalized) electrons from the metal grid and on the other hand as a contact or potential tap, by which a measurement of itself due to the above-described release or uptake of electrons on the metallic conductor ( Au or Pt) emerging potential is possible.

By the electrochemical compound of ME and RE over a cation and in particular a sodium ion-conducting solid electrolyte such as Na-β- and or -β '' -. Al ₂ O ₃ having the composition Na ₂ O · nAl ₂ O ₍₃ n = 5 ... 11 where n = 5 ... 7 = β '' and n = 7 ... 11 = β) or the compound known as Nasicon having the general composition Na _{1 + x} Zr ₂ Si _x P _{3 x} O ₁₂ (0 <x <3), a cell reaction according to equation (3) can be formulated for the sensor cell: Na ₂ CO ₃ + 2SiO ₂ ⇌ Na ₂ Si ₂ O ₅ + CO ₂ (3)

The cations and, in particular, sodium ion-conducting solid electrolyte fulfill the function of an electrolyte bridge or a current key, which connects the two half-cells (RE and ME), but only allows the transport of sodium ions between the two half-cells. In the 1 listed Na ⁺ conductive solid electrolytes are characterized by the fact that their ionic conductivity above 250 ° C allows rapid equilibration between the two electrodes and that they have a negligible amount of electronic conductivity. In principle, however, other cations may also use conductive solid electrolytes for this purpose.

The activity dependence of the electrical equilibrium potentials φ _{0 of} the redox reactions taking place at the electrodes can be described by the Nernst equation according to equation (4). R is the universal gas constant, T the absolute temperature in Kelvin, F the Faraday constant, z _e the equivalent number of electrons exchanged per formula conversion, v _i the stoichiometric factor of component i per formula conversion, a _i the activity of component i and φ ₀₀ the standard potential.

Under The activity can generally be considered an efficacy factor understand the corrected concentration of a component, the Activity of gaseous components under normal conditions their partial pressure p and the activity pure, crystalline solids is one.

The measurable cell voltage (or EMF) of the potentiometric CO ₂ sensor 1 , results from the difference between the two electrode or half cell potentials. For the half-cell reactions of Equations (1) and (2), for example, the cell voltage of the sensor is calculated from the difference of Equation (5) and Equation (6) with the simplification of Equation (7). Considering further that due to the planar arrangement of the electrodes, the oxygen activity at the measuring and reference electrodes is the same, the sensor cell voltage corresponding to the whole cell reaction from equation (3) can be described by equation (8):

(entspricht dem Anstieg der durch Gleichung 8 wiedergegebenen Sensor-Kennliniengleichung in einem logarithmischen Koordinatensystem) beider Elektrodenreaktionen auf eine Änderung des O₂ Partialdruck gleich ist und damit nur eine Abhängigkeit vom CO₂ Partialdruck gemessen wird. 2 verbildlicht dieses grundlegende Prinzip noch einmal: Diese Fig. zeigt das Prinzip der potentiometrischen Messung des CO₂ Partialdrucks über die Differenz des CO₂ und O₂ abhängigen Gleichgewichtspotentials an der Messelektrode und dem O₂ abhängigen Gleichgewichtspotential an der Referenzelektrode (U = gemessene Sensor- oder Zellspannung oder EMK in bspw. mV).The measurable by means of the potentiometric sensor CO ₂ concentration or the partial pressure thus results from the change in the equilibrium potentials in a change in CO ₂ concentration in that the sensitivity

(Corresponds to the increase in the reproduced by equation 8 sensor-characteristic equation in a logarithmic coordinate system) of both electrode reactions to a change in the O ₂ partial pressure is equal and thus only a function of CO ₂ partial pressure is measured. 2 This basic principle illustrates once more: This figure shows the principle of the potentiometric measurement of CO ₂ partial pressure on the difference of the CO ₂ and O _2- dependent equilibrium potential at the measuring electrode and the O _2- dependent equilibrium potential at the reference electrode (U = measured sensor or Cell voltage or emf in, for example, mV).

The essential basic idea of the present invention now lies in utilizing the reference electrode potential which is dependent on the partial pressure of one of the gas constituents (in particular: on the oxygen partial pressure) in order to allow a combined measurement of the two gas components (in particular of CO ₂ and O ₂ ) with exactly one potentiometric sensor element enable. This is inventively realized in that a third electrode (the reference electrode) is introduced, the equilibrium potential (reference electrode potential φ BE / 0 , see. 3 ) is independent of the partial pressures of the first and second gas constituents present in the measuring gas or the gas sample (cf. 3 ). 3 thus shows the principle of the combined potentiometric measurement according to the invention of the concentration or of the partial pressure of the first and of the second gas component using the example of CO ₂ and O ₂ .

The Independence of the equilibrium potential of the third Electrode or the reference electrode of the existing gas in the measurement Partialpressions of the two gas components is preferably by a gas-tight sealing of the reference electrode against the Measuring gas space in which the gas sample is located ensured.

All three electrodes (reference, measuring and reference electrodes) are in surface contact with one and the same solid electrolyte or solid electrolyte body. The solid electrolyte (the body) may be formed as a single, cations conductive layer, this may be also a more cations conductive layers (eg. As a Na ⁺ ions and K ⁺ ions conductive layer) act complete layer system.

By providing the reference electrode BE, three cell voltages can thus be measured within the scope of the present invention. The decisive factor here is that only one cation-conducting (eg sodium ion-conducting) solid electrolyte layer is required between the electrodes (all three electrodes are thus preferably spaced) from each other - arranged in a planar manner with one and the same solid electrolyte layer). The electrochemical equilibrium potential of the reference electrode is not a function of the partial pressure of the first and no function of the partial pressure of the second gas component of the gas sample or the measuring gas. The measured potential difference between the reference electrode and the reference electrode U _RE-BE is thus at in 3 The second measured sensor voltage of the example shown (potential difference between measuring electrode and reference electrode U _ME-BE ) is a function of the oxygen partial _pressure and the carbon dioxide _{partial pressure} . The third measured sensor voltage (potential difference between measuring electrode and reference electrode U _{ME- RE} ) represents only a function of the carbon dioxide partial pressure in the example shown.

On this basis, according to the invention, for example, the CO ₂ and the O ₂ concentration in gases can be measured by means of only one potentiometric sensor which permits a combined CO ₂ and O ₂ measurement, the sensor being used, for example, as a simple and reliable measuring system can be built from a voltmeter and, for example, two relays, which is characterized by a high degree of miniaturization and ease of manufacture.

The selectivity of the gas-sensitive materials from the specific gas constituents is ideally represented by the redox reaction, such as in equations (1) and (2), and prepared by a suitable choice of material. The determination of the O ₂ and CO ₂ partial pressure via the different voltages at the sensor (see 5 ) can be done, for example, via a previous calibration of the sensor with test gases of different concentrations of the gas components to be analyzed (see 5 ).

für das Beispiel in Gleichung (8)) für jeweils ein Elektrodenpaar des Sensors bestimmt werden. Die Konzentration der Gasbestandteile wird folglich wärend einer Messung einer Gasprobe aus den im Messsystem hinterlegten Funktionen für den Zusammenhang zwischen Sensorspannung und Partialdruck und die hierfür im vornherein bestimmten Parameter A und B (siehe 5) ermittelt.According to the characteristic equation (8), during a calibration the parameters A (corresponding to U0 for the example in equation (8)) and B (corresponding to U0 for the example in equation (8)) can be obtained from the measured voltages at different gas concentrations

for the example in equation (8)) for each pair of electrodes of the sensor. The concentration of the gas constituents is consequently during a measurement of a gas sample from the functions stored in the measuring system for the relationship between the sensor voltage and the partial pressure and the parameters A and B (see 5 ).

4 shows an example of the measurable sensor voltages at different CO ₂ and O ₂ concentrations.

5 shows an example of a calibration curve for the determination of the parameters A and B of the sensor characteristic (eg U _ME-RE )

• (a) der CO₂ Partialdruck abhängigen Sensorspannung
• (b) der CO₂ und O₂ Partialdruck abhängigen Sensorspannung und (c) der O₂ Partialdruck abhängigen Sensorspannung.

6 shows an example of a possible circuit of the electrodes of the sensor for determining, for example.

• (a) the CO ₂ partial pressure-dependent sensor voltage
• (b) the CO ₂ and O ₂ partial pressure dependent sensor voltage and (c) the O ₂ partial pressure dependent sensor voltage.

The following 7 and 8th show two concrete embodiments of the present invention using the example of combined CO ₂ / O ₂ sensors.

7 shows a first embodiment with a cation-conducting solid electrolyte 2 , On a surface (in the 7 top surface) of the solid electrolyte 2 are laterally spaced from each other and immediately adjacent to the solid electrolyte 2 the three electrodes, namely the measuring electrode 3 , the reference electrode 4 and the reference electrode 5 arranged. All three electrodes are in surface contact with the solid electrolyte 2 and are as flat, planar layer portions with a thickness of about 50 nm to 500 microns and preferably ~ 50 microns on the surface of the solid electrolyte 2 arranged. The thickness of the solid electrolyte 2 here is about 20-100 microns.

On its underside, the measuring electrode 3 an electrically in contact with her, directly to the solid electrolyte 2 adjacently arranged electrode grid 3b on, with that at the measuring electrode 3 applied electrical equilibrium potential via a with the measuring electrode grid 3b connected, electrically conductive measuring electrode lead 3a can be derived to an input of a voltmeter.

An appropriate structure, the reference electrode 4 on: Between it and the solid electrolyte or adjacent to its bottom and to the top of the solid electrolyte 2 lies a reference electrode grid 4b , by means of which the equilibrium potential of the reference electrode 4 via one with the reference electrode grid 4b connected reference electrode lead 4a can be derived to a second input of the above-described voltmeter. Thus, the electrical potential difference between the measuring and reference electrodes is applied to the two inputs of the voltmeter.

In the example shown is between the measuring electrode 3 and the reference electrode 4 and spaced and electrically isolated from these two electrodes, the reference electrode 5 arranged. This also stands, like the measuring electrode 3 and the reference electrode 4 , in surface contact with the solid electrolyte 2 , Again, between the reference electrode 5 and the surface of the solid electrolyte 2 a reference electrode grid 5b formed, that, by connection to a reference electrode lead 5a , for discharging the at the reference electrode 5 applied electrical equilibrium potential.

On her the solid electrolyte 2 opposite surface is the reference electrode 5 (unlike the electrodes 3 and 4 ) completely from a gas-impermeable layer 6 (For example, a glass layer) covered, so that the reference electrode 5 including the reference electrode grid 5b completely, ie gas-tight, in the composite of solid electrolyte 2 and gas-permeable layer material 6 is encapsulated (the lead out of the lead portion 5a is designed so that this gas-tightness is ensured).

As described in more detail below, the material of the measuring electrode 3 gas-sensitive for oxygen and carbon dioxide and the material of the reference electrode 4 gas-sensitive for oxygen. A measurement gas sensitivity of the reference electrode 5 is, due to the complete encapsulation, not given here.

The above-described sensor is in a sample gas space 1 introduced, in which the oxygen and carbon dioxide containing gas sample P is present: The gas-sensitive material of the measuring electrode 3 and the reference electrode 4 (as well as the reference electrode 5 opposite surface of the gas-impermeable layer 6 ) is thus in gas contact with the gas sample P in the sample gas space 1 ,

7 shows an embodiment of the invention, wherein in a potentiometric CO ₂ solid electrolyte sensor additionally a reference electrode 5 has been applied to a cations conductive solid electrolyte layer (for example via screen printing) with surface contact and then this reference electrode using a gas impermeable executed layer (layer 6 ), which may be one or more sintered glass single layers, is gas tight against the sample gas atmosphere 1 has been sealed.

• ME – Messelektrodenstruktur,
• BE – Bezugselektrodenstruktur,
• RE – Referenzelektrodenstruktur und
• 2: Kationen- oder, im speziellen, Natriumionen leitender Festelektrolyt aus Nasicon oder Na-beta-Al₂O₃ (in Form einer gedruckten Schicht, oder eines verpressten Formkörpers, oder einer über eine Festkörperreaktion ausgebildete Kation-beta-Al₂O₃ Diffussionsschicht in bzw. auf einem Al₂O₃-Substrat).
• Weitergehend:
• 3a: Messelektrodenzuleitung (in Form eines Drahtes und/oder einer gedruckten Schicht),
• 3b: Messelektrodengitter (in Form einer gedruckten teilweise durchlässigen Schicht aus Au oder Pt),
• 3: Elektrochemisch aktive Messelektrode mit gassensitivem Material wie z. B. Na₂CO₃ und Au (bzw. Pt) (in Form einer gedruckten Schicht und/oder eines verpressten Formkörpers),
• 5a: Bezugselektrodenzuleitung (in Form eine Drahtes und/oder einer gedruckten Schicht),
• 6: Gasimpermeable Bezugselektrodenabdeckung aus bspw. einer oder mehreren gesinterten Glasschichten (in Form einer vorzugsweise gedruckten Schicht),
• 5: elektrochemisch aktive Bezugselektrode in Form einer gedruckten Schicht oder eines verpressten Formkörpers oder in Form einer sich selbst ausbildenden Reaktionszone zwischen dem Kationen leitenden Festelektrolyt und Bestandteilen der Bezugselektrodenabdeckung.
• 5b: Bezugselektrodengitter (in Form einer teilweise durchlässigen Schicht aus Au oder Pt),
• 4a: Referenzelektrodenzuleitung (in Form eines Drahtes und/oder einer gedruckten Schicht),
• 4b: Referenzelektrodengitter (in Form einer gedruckten teilweise durchlässigen Schicht aus Au oder Pt),
• 4: Elektrochemisch aktive Referenzelektrode mit gassensitivem Material wie bspw. einer Mischung aus Na₂Si₂O₅, SiO₂ und Au (bzw. Pt) (in Form einer gedruckten Schicht und/oder eines verpressten Formkörpers).

The configuration can be as follows:

• ME measuring electrode structure,
• BE - reference electrode structure,
• RE reference electrode structure and
• 2 Cation or, in particular, sodium ion conductive solid electrolyte from Nasicon or Na-beta-Al ₂ O ₃ (in the form of a printed layer, or a pressed molded article, or formed via a solid state reaction cation beta-Al ₂ O ₃ diffusion layer in or on an Al ₂ O ₃ substrate).
• Proceeding:
• 3a : Measuring electrode lead (in the form of a wire and / or a printed layer),
• 3b : Measuring electrode grid (in the form of a printed partially transparent layer of Au or Pt),
• 3 : Electrochemically active measuring electrode with gas-sensitive material such. B. Na ₂ CO ₃ and Au (or Pt) (in the form of a printed layer and / or a compressed molding),
• 5a Reference electrode lead (in the form of a wire and / or a printed layer),
• 6 : Gas impermeable reference electrode cover of, for example, one or more sintered glass layers (in the form of a preferably printed layer),
• 5 Reference: Electrochemically active reference electrode in the form of a printed layer or a pressed shaped body or in the form of a self-forming reaction zone between the cation-conducting solid electrolyte and constituents of the reference electrode cover.
• 5b : Reference electrode grid (in the form of a partially transparent layer of Au or Pt),
• 4a Reference electrode lead (in the form of a wire and / or a printed layer),
• 4b : Reference electrode grid (in the form of a printed partially transparent layer of Au or Pt),
• 4 Electrochemically active reference electrode with gas-sensitive material such as, for example, a mixture of Na ₂ Si ₂ O ₅ , SiO ₂ and Au (or Pt) (in the form of a printed layer and / or a compressed molding).

To the electrochemically active reference electrode 5 : The involved in the redox reaction phases of the electrochemically active reference electrode layer can be registered in the production as an electrode layer by selective admixture in one of the glass layers or in situ z. B. form by crystallization of such phases from the glass of the cover layers. In this case, the redox reaction of equation (2) can take place at the reference electrode due to an electrochemically active reference electrode layer of Na ₂ Si ₂ O ₅ and SiO ₂ , which forms by a suitable heat treatment in the production.

For this There must be enough gold in the electrode to get the metallic one Conductivity of the composite material to achieve (in the Rule ≥ 30 vol.%)

The "self-education" of Electrochemically active layer can through a large interface between the first glass cover layer (if necessary, these reactive components added), gold electrode and electrolyte, as well as by a suitable heat treatment in the manufacture of the sensor be achieved.

Without admixture of Au or Pt, measurable results were obtained in one of the glass cover layers, the Au electrode being present as a 0.1 μm <x <20 μm thick, partially permeable layer between the glass cover and the electrolyte. Advantageously, 20-50 Ma.% Au may be present in an additional reactive glass layer of, for example, SiO ₂ and Na ₂ Si ₂ O ₅ (see also reference electrode).

in principle However, other material systems are also reasonable, s. ff.

The electrode grid consists of an Au paste which may contain small amounts of an adhesion-promoting component to increase the adhesion between electrolyte and electrode. The electrode grid layer is preferably printed on the electrolyte layer with a thickness of 0.1 microns <x <20 microns and has recesses and / or a porosity between the electrode layer and electrolyte to a contact between gas-sensitive material and electrolyte.

The electrode layers preferably have a content of 20-50 Ma.% Au and are printed on the electrode grid layer with a thickness of 0.1 μm <x <100 μm. Previously realized electrodes each had an area of 22 mm ² to 2 mm ^2, depending on the sensor design. Conceivable electrodes are between 5000 mm ² and 0.05 mm ²

The individual layers and components of the sensor can According to the invention are prepared so that they in single steps and / or together after printing or Pressing at elevated temperatures (i.r., R. ≥ 580 ° C, ≤ 1350 ° C) be sintered to a good structural strength and connection or a good electrical or ionic conductivity of the layers and / or to be able to train networks.

The cation-conducting solid electrolyte may be applied in the form of a printed layer on a substrate of, for example, Al ₂ O ₃ , which is advantageous in terms of miniaturization of the sensor, since the substrate (not in FIG 7 shown), the dimensions of the cation-conducting solid electrolyte and the electrodes applied thereto can be decisively reduced or consist of a solid which is produced by the pressing and sintering of a starting powder (consisting of the actual electrolyte material or a precursor material). Such a shaped body of a cation-conducting solid electrolyte can itself be used as a supporting element for the other functional layers. A specific form of the solid electrolyte coating with a cation-conducting solid electrolyte layer such as Na-beta-Al ₂ O ₃ , is that the solid electrolyte layer by a solid state reaction with the carbonate or the oxide of the cation such as Na ₂ CO ₃ with an Al ₂ O ₃ substrate, which preferably has an Al ₂ O ₃ content of ≥ 99 wt .-%, is produced to form a Na-beta-Al ₂ O ₃ layer in the substrate. See also Ionics, 3 (1997) O. Schaef, T. Widmer and U. Guth - In-Situ Formation of Thin Film Like β '' - Alumina Layers on alumina-alumina Substrates, pages 277-281 ,

• – Die Gruppe der Beta-Aluminat Verbindungen mit den Kationen: Na⁺, K⁺, Li⁺, Mg²⁺, Ca²⁺, Ag⁺, Pb²⁺, Sr²⁺ und Ba²⁺ wie bspw. Na-beta-Al₂O₃ (Na₂O nAl₂O₃ mit n = 5...11 wobei n = 5...7 = β'' und n = 7...11 = β) bzw. Naβ/β''-Al₂O₃
• – Die als „super ionic conductor” bezeichneten Verbindungen der Zusammensetzung M_1+xZr₂Si_xP_3-xO₁₂ (0 ≤ x ≤ 3) mit den Kationen M = Li, Na, K wie bspw. Nasicon (Na_1+xZr₂Si_xP_3-xO₁₂ (0 ≤ x ≤ 3))

As cations conductive solid electrolyte for use in the combined potentiometric CO ₂ -O ₂ solid electrolyte sensor z. B. in question:

• The group of beta-aluminate compounds with the cations Na ⁺ , K ⁺ , Li ⁺ , Mg ²⁺ , Ca ²⁺ , Ag ⁺ , Pb ²⁺ , Sr ²⁺ and Ba ^2+, such as Na-beta. Al ₂ O ₃ (Na ₂ O n Al ₂ O ₃ with n = 5 ... 11 where n = 5 ... 7 = β '' and n = 7 ... 11 = β) or Naβ / β '' -Al ₂ O ₃
• The compounds designated as "super ionic conductors" of the composition M _{1 + x} Zr ₂ Si _x P _{3 -x} O ₁₂ (0≤x≤3) with the cations M = Li, Na, K, such as, for example, Nasicon (Na _{1 + x} Zr ₂ Si _x P _{3 -x} O ₁₂ (0 ≤ x ≤ 3))

As a gas-sensitive material on the measuring electrode, it is generally possible to use mixtures of alkali and / or alkaline earth metal carbonates with Au and / or Pt. Also additives of rare earth oxides or carbonates are known from the literature (see also M. Yamauchi, Y. Itagakia, H. Aono and Y. Sadaoka, "Reactivity and stability of rare earth oxides - Li 2 CO 3 mixtures", J. Ot European Ceramic Society, 28 (2008), 27-34 ).

The Gas-sensitive measuring electrode layer can be in the form of a printed Layer and or a pressed molding with the Cations conductive solid electrolyte are used in contact. at For example, a gas-sensitive measuring electrode layer containing lithium carbonate and a sodium ion-conducting solid electrolyte, it depends on the Phase boundary carbonate / electrolyte to exchange reactions, the one electrochemical coupling of the half-cells over the electrolyte allowed.

As a gas-sensitive material on the reference electrode, mixtures of SiO ₂ and Na ₂ Si ₂ O ₅ with Au or Pt or TiO ₂ and Na ₂ Ti ₃ O ₇ (and / or Na ₂ Ti ₆ O ₁₃ ) with Au or Pt are common , However, other less commonly used materials such as FeO + NiO borate glass in contact with FeNi (48) alloys or Na _x WO ₃ (0.3 <x ≤ 1) are also known.

The Gas-sensitive reference electrode layer may also be in the form of a printed Layer and or a pressed molding with the Cations conductive solid electrolyte are brought into contact.

In principle, all materials which can be used on the reference electrode or measuring electrode are considered as electrochemically active reference electrode materials. When using these materials, it should be noted that these materials all form an equilibrium potential in equilibrium with a gaseous component (CO ₂ and or O ₂ ). Due to the equilibration, this may lead to a gas pressure below the gas-impermeable reference electrode cover, whereby the reference electrode cover possibly defects such. B. can form cracks. However, one application of these materials is quite conceivable taking into account this circumstance, ie by adaptation of the reference electrode cover or selection of materials which form the lowest possible equilibrium partial pressure of their gaseous reaction products (see below).

In order to prevent the formation of a gas pressure at the reference electrode in advance, it makes sense to select the reference electrode material so that the equilibrium potential forming is independent of a gas. An example of this is the reaction equation for a redox reaction on the BE given in equation (9): Ag ₂ SO ₄ + 2Na ⁺ + 2e ^- ⇌ 2Ag + Na ₂ SO ₄ (9)

There it by introducing additional chemical elements (For example, Ag and S in equation (9)) to unwanted side reactions between the cation-conducting solid electrolyte and the electrochemical active components and the reference electrode cover can come is an adaptation of cover layer and electrochemically active Reference electrode layer makes sense. For this purpose, for example, additional layers between the gas impermeable covering layer and the electrochemical active reference electrode layer can be applied. Otherwise could be unwanted reactions between components of the electrochemically active reference electrode layer and the cover layer to a long-lasting drift of the reference electrode potential to lead.

An adaptation of the layers to one another can be done in a preferred variant by forming an electrochemically active reference electrode layer in-situ at the phase boundary between the cover layer, the solid electrolyte and the Au or Pt of the electrode grid, with suitable choice of the material of the cover layer (s) can. This can be z. B. happen by the crystallization of Na silicate phases in the heat treatment of the layers of the sensor in the glass. Since form as the potential-forming phase of the existing constituents of the cover layer and the solid electrolyte further side reactions can be neglected. In addition, a self-forming reference electrode layer makes it possible to decisively simplify the design of the combined CO ₂ -O ₂ sensor, since only one electrode grid has to be printed and then sealed gas-tight. Since the above-described independent formation of the electrochemically active reference electrode layer may take place very slowly, in a further preferred variant, the electrochemically active reference electrode layer in the form of a mixture of sodium silicate, for example. Na ₂ Si ₂ O ₅ , silica SiO ₂ and Au (or Pt ) are already applied to the electrode grid of the reference electrode. The Bezugselektrodenabdeckschicht can in this case also consist of a sodium silicate, which has no electrically conductive components and possibly no further electrochemically active ingredients. This reference electrode covering layer can then be sealed against the measuring gas atmosphere by further layers of, for example, a glass.

When Gas impermeable reference electrode cover are preferably one or more layers of glass on the electrochemically active reference electrode layer deposited in the form of a preferably printed layer and individually and / or collectively after printing at elevated temperatures (≥ 450 ° C, ≤ 1000 ° C) gas-tight sintered. In another variant, the cation can also be conductive Solid electrolyte material can be used as a reference electrode cover. For a successful gas-tight encapsulation of the reference electrode It must also be ensured that the solid electrolyte layer has no continuous porosity, so that no gas diffusion from locations outside the reference electrode cover to the electrochemically active reference electrode layer occurs.

Another example of the present invention shows 8th : The combined CO ₂ / O ₂ sensor shown there is basically like the one in 7 shown sensor, so that only the differences to in the following 7 be described sensor described.

Instead of the reference electrode structure 5 . 5a . 5b and 6 has the in 8th shown sensor between the measuring electrode structure 3 . 3a . 3b and the reference electrode structure 4 . 4a . 4b and spaced from these two electrode structures one in surface contact with the solid electrolyte 2 standing, arranged on the surface O1 barrier layer of an inert material 9 (here: glass). This prevents short-circuit currents on the surface of the solid electrolyte 2 and an electrode contact between the electrodes 3 and 4 due to "material leakage" from the electrodes 3 . 4 , The on top of the solid electrolyte 2 formed measuring electrode structure and reference electrode structure (as well as the barrier layer 9 ) are in gas contact with the sample gas space 1 in which the gas sample P to be measured is present.

On the opposite side of this top O1 (bottom O2 of the solid electrolyte 2 ) is in surface contact with the bottom O2 the reference electrode structure 5 . 5a . 5b arranged.

The solid electrolyte 2 is formed between the top O1 and the bottom O2 as a gas-tight layer so that the measurement gas space at the top O1 gas-tight from that at the bottom O2 of the solid electrolyte 2 trained reference gas space 7 is separated. The separation of sample gas space and reference gas space is thus realized by the gas-tight solid electrolyte itself. The measuring electrode structure 3 . 3a . 3b and the reference electrode structure 4 . 4a . 4b are thus in gas contact with the gas sample space or sample gas space 1 (but not with the reference gas space 7 ), the reference electrode structure 5 . 5a . 5b is in gas contact with the reference gas space 7 (but not with the sample gas chamber 1 ).

The reference electrode structure 5 . 5a . 5b is here formed analogously to the reference electrode structure, thus has in addition to a gas-sensitive material 5 (gas-sensitive for oxygen) an electrode grid 5b and a reference electrode lead 5a on. The materials of each element will be described below.

At the in 5 Thus, the example shown is the reference electrode 5 like the oxygen-sensitive reference electrode 4 executed and (gas-tight separated from the sample gas chamber 1 ) in gas contact with a reference gas atmosphere (in the simplest case room air) brought. The advantage of this embodiment is that because of the same reference electrode and reference electrode composition, a very simple production is ensured, since no additional gas-tight sealed electrode (see reference electrode in US Pat 4 ) must be applied. As a disadvantage of this embodiment, the gas-tight from the sample gas space 1 to be separated reference gas space 7 to see. This gas separation can, for example, by a glass seal 8th respectively.

The barrier layer 9 is at the 7 shown sensor not necessary because there the layer 6 this function takes over.

• ME – Messelektrodenstruktur,
• BE – Bezugselektrodenstruktur,
• RE – Referenzelektrodenstruktur,
• 2: Kationen- oder im speziellen Natriumionen leitender Festelektrolyt aus Nasicon oder Na-beta-Al₂O₃ (in Form einer gedruckten Schicht oder eines verpressten Formkörpers, oder einer über eine Festkörperreaktion ausgebildeten Diffussionsschicht.)
• 9: inerte Barriereschicht aus bspw. einem Glas, durch welche Kurzschlussströme auf der Oberfläche des Festelektrolyten und ein Elektrodenkontakt verhindert werden,
• 3a: Messelektrodenzuleitung (in Form eines Drahtes und/oder einer gedruckten Schicht),
• 3b: Messelektrodengitter (in Form einer gedruckten teilweise durchlässigen Schicht aus Au oder Pt),
• 3: Elektrochemisch aktive Messelektrode mit gassensitivem Material wie z. B. Na₂CO₃ und Au (bzw. Pt) (in Form einer gedruckten Schicht und/oder eines verpressten Formkörpers),
• 5a: Bezugselektrodenzuleitung (in Form eines Drahtes und/oder einer gedruckten Schicht),
• 5b: Bezugselektrodengitter (in Form einer gedruckten teilweise durchlässigen Schicht aus Au oder Pt),
• 5: Elektrochemisch aktive Bezugselektrode mit gassensitivem Material wie z. B. einer Mischung aus Na₂Si₂O₅, SiO₂ und Au (bzw. Pt) (in Form einer gedruckten Schicht und/oder eines verpressten Formkörpers),
• 4a: Referenzelektrodenzuleitung (in Form eines Drahtes und/oder einer gedruckten Schicht),
• 4b: Referenzelektrodengitter (in Form einer gedruckten teilweise durchlässigen Schicht aus Au oder Pt),
• 4: Elektrochemisch aktive Referenzelektrode mit gassensitivem Material wie z. B. einer Mischung aus Na₂Si₂O₅, SiO₂ und Au (bzw. Pt) (in Form einer gedruckten Schicht und/oder eines verpressten Formkörpers).

The individual elements of in 8th shown sensor may be formed as follows:

• ME measuring electrode structure,
• BE - reference electrode structure,
• RE reference electrode structure,
• 2 Nasicon or Na-beta-Al ₂ O ₃ cationic or, in particular, sodium ion-conducting solid electrolyte (in the form of a printed layer or a compressed molding, or a diffusion layer formed via a solid-state reaction)
• 9 inert barrier layer of, for example, a glass, by which short-circuit currents on the surface of the solid electrolyte and an electrode contact are prevented,
• 3a : Measuring electrode lead (in the form of a wire and / or a printed layer),
• 3b : Measuring electrode grid (in the form of a printed partially transparent layer of Au or Pt),
• 3 : Electrochemically active measuring electrode with gas-sensitive material such. B. Na ₂ CO ₃ and Au (or Pt) (in the form of a printed layer and / or a compressed molding),
• 5a Reference electrode lead (in the form of a wire and / or a printed layer),
• 5b : Reference electrode grid (in the form of a printed partially transmissive layer of Au or Pt),
• 5 : Electrochemically active reference electrode with gas-sensitive material such. B. a mixture of Na ₂ Si ₂ O ₅ , SiO ₂ and Au (or Pt) (in the form of a printed layer and / or a compressed molding),
• 4a Reference electrode lead (in the form of a wire and / or a printed layer),
• 4b : Reference electrode grid (in the form of a printed partially transparent layer of Au or Pt),
• 4 : Electrochemically active reference electrode with gas-sensitive material such. B. a mixture of Na ₂ Si ₂ O ₅ , SiO ₂ and Au (or Pt) (in the form of a printed layer and / or a compressed molding).

und die Ansprechzeit von Vorteil.In principle, the sensors listed here already work from temperatures of ~ 300 ° C, since the currently known cation conductive solid electrolyte systems from 250 ° C have a sufficient ionic conductivity. However, a higher operating temperature (> 400 ° C) is appropriate in terms of sensor sensitivity

and the response time of advantage.

• • ein Heizelement vorzugsweise in Form eines Widerstandsheizers der über eine Drucktechnologie auf die Rückseite eines tragenden Substrats oder auf die Rückseite eines Festelektrolytformkörpers aufgebracht wird (letzteres bedingt eine elektrisch und chemisch isolierende Trennschicht zw. Heizelement und Festelektrolytschicht um chemische bzw. elektrochemische Reaktionen zu vermeiden),
• • ein Heizelement in Form eines Widerstandsheizers als gewickelter Draht der um das Sensorelement oder den das Sensorelement beinhaltenden Probenraum angebracht ist und/oder
• • ein indirekt eingetragener Wärmestrom ausgenutzt wird, um den Sensor zu Heizen, z. B. durch die Anbringung des Sensors in der Nähe heißer Elemente wie bspw. einen porösen Nachbrenner für die Abgasnachbehandlung.

The heating of the sensor advantageously provided for the operation of the combined CO ₂ -O ₂ sensor can take place within the scope of the invention in that

• A heating element, preferably in the form of a resistance heater, which is applied to the back side of a supporting substrate or to the rear side of a solid electrolyte molded body via a printing technology (the latter requires an electrically and chemically insulating separating layer between heating element and solid electrolyte layer in order to avoid chemical or electrochemical reactions),
• A heating element in the form of a resistance heater is applied as a wound wire around the sample element or the sample space containing the sensor element and / or
• • an indirectly registered heat flow is used to heat the sensor, eg. B. by the attachment of the sensor in the vicinity of hot elements such as, for example, a porous afterburner for exhaust aftertreatment.

The combined potentiometric CO ₂ -O ₂ sensor on which the invention is based is distinguished by its simple construction and the good miniaturization following from the measuring principle with the aid of high-throughput techniques, such as, for example, B. the screen printing. A corresponding 7 on a supporting substrate of 99% Al ₂ O ₃ ceramic, with integrated heating element on the opposite side of the electrodes and Nasicon solid electrolyte layer could be made entirely by screen printing and shows the feasibility of a cost-effective mass production of the combined CO ₂ -O ₂ sensor, the it does not exist in this form yet.

However, the introduction of a measuring gas-independent reference electrode results in further decisive economic and scientifically valuable advantages:
The three measurement signals (voltages) allow statements about changes to each individual electrode such as 3 evident. This results in addition to the measurement of the change in the oxygen partial pressure (over U _BE-RE ) and the ability to track unwanted changes to the electrodes such as drifting due to side reactions or disturbances of the equilibrium potential of an electrode. If, for example, two measuring voltages change identically without the third one changing (with the gas composition remaining the same), it can be concluded that the electrode, which is not involved in the stable measuring voltage, changes over time. In this way, the three electrodes of the combined CO ₂ -O ₂ sensor make it possible to gain new insights into the chemical or electrochemical stability of electrode materials compared to environmental variables (such as temperature, reactions with gases such as H ₂ O) and electrode materials their performance (eg response time or cross-sensitivity) for use in the potentiometric sensor to characterize. This is immensely advantageous with regard to the large number of possible electrode materials.

The three measuring signals (voltages) also allow a self-diagnosis or simple calibration routine of the measuring system. Thus, a change in the signal of the sensor voltage between the measuring and reference electrode due to a change in the CO ₂ concentration between the reference and measuring electrode to the same extent from, otherwise there is a disturbance in the system.

Today's CO ₂ solid electrolyte sensors are calibrated in practical application via a one-point calibration. In this case, room air is used as the calibration gas and the CO ₂ concentration is assumed to be at a constant value of, for example, 380 ppm. The slope of the sensor characteristic (ie the sensitivity) is assumed to be constant over the entire operating time and the offset voltage is corrected. Since it is known that the CO ₂ concentration in the atmosphere is subject to certain fluctuations and under certain circumstances, the slope of the sensor characteristic (ie the sensitivity) can change, a 2-point calibration is crucial for the accuracy of such sensors. Due to the three measuring signals (voltages) of the combined potentiometric CO ₂ -O ₂ sensor, it is basically possible to perform such a 2 (or more) -point calibration of the measuring system only by the changes in the O ₂ partial pressure in the measuring gas. As a result, the sensitivity of reference and measuring electrode can be measured independently of each other and additionally compensate for a possible oxygen interference of the CO ₂ measurement.

idealerweise an beiden Elektroden gleich ist (s. a. Gleichung (5) und (6)). Dies stimmt jedoch nicht immer mit der Realität überein, so dass immer eine gewisse Sauerstoffquerempfindlichkeit dieser Sensoren beobachtet wird. Durch die oben erläuterte Kalibrierfähigkeit des der Erfindung zu Grunde liegenden CO₂-O₂-Sensors kann eine vollkommen neue Qualität der Messung mittels dieses potentiometrischen Sensors, durch die 2-Punkt Kalibrierung und anschließende rechnerische Kompensation der unterschiedlichen Elektrodenempfindlichkeiten erzielt werden. Eine ähnliche Kalibrierung kann auch bspw. in der Atemgasanalyse erfolgen, indem während der Messung die veränderliche CO₂ und O₂ Konzentration über einen Bypass geleitet wird, in dem sich ein CO₂-Filter befindet, der die Änderungen der CO₂ Konzentration während der Ein- und Ausatmung glättet, so dass nur die Veränderung der O₂ Konzentration am Sensor erkannt wird. In Kombination mit einem O₂ Sensor der keine Kalibrierung benötigt wie bspw. eine Grenzstromsonde kann dann das System auf Grund der bekannten Änderung der O₂ Konzentration kalibriert werden. Hierdurch wird prinzipiell eine genauere Messung ermöglicht.For this purpose, for example, the combined CO ₂ -O ₂ sensor can be installed in or connected to an oxygen pumping cell in such a way that a known change in the oxygen partial pressure in the pump cell (eg platinum electrodes on a YSZ solid electrolyte) is achieved by a defined current supply Measuring room of the combined CO ₂ -O ₂ sensor is caused. Since both the measuring and the reference electrode potential changes dependent on the oxygen partial pressure, the electrode sensitivity can be measured by changing the sensor voltage between the measuring and reference electrode or reference and reference electrode. As mentioned above, it is assumed for the potentiometric sensors that the change in the equilibrium potential as a function of the oxygen partial pressure

ideally equal at both electrodes (see also equation (5) and (6)). However, this does not always coincide with the reality, so that always a certain oxygen interference of these sensors is observed. The calibration capability of the CO ₂ -O ₂ sensor on which the invention is based makes it possible to achieve a completely new quality of measurement by means of this potentiometric sensor, by the 2-point calibration and subsequent computational compensation of the different electrode sensitivities. A similar calibration can also be done, for example. In the breathing gas analysis by the variable CO ₂ and O ₂ concentration is passed through a bypass during the measurement, in which there is a CO ₂ filter, the changes in CO ₂ concentration during the on - And exhalation smoothes, so that only the change in the O ₂ concentration is detected on the sensor. In combination with an O ₂ sensor that does not require calibration, such as a limit current probe, the system can then be calibrated based on the known change in O ₂ concentration. As a result, a more accurate measurement is possible in principle.

in the Under the invention, a variety of different cations conductive Solid electrolytes, precious metals, materials for gas-sensitive Layer on the measuring electrode, materials for the gas-sensitive Layer on the reference electrode and combinations of these materials be used.

As already mentioned, not only the gas constituents O ₂ and CO ₂ can be determined with the present invention, but also (for example when using NaNO ₃ as a gas-sensitive layer at the measuring electrode or an additional electrode, concentrations of NO ₂ can be measured. Thus, for example, in the context of the invention, the first gas component O ₂ and the second gas component NO ₂ can be determined.

By applying more than three electrodes (for example, by applying an additional electrode with NaNO ₃ as a gas sensitive layer in the in 7 and 8th In the examples shown, it is also possible to measure three gases with regard to their concentration in a gas sample.

One inventive potentiometric sensor can For example, in the room air monitoring (for example in offices or in the interior of vehicles), in medical technology (for example in respiratory gas analyzers), in consumer electronics (for example in protection technology or with oven controls; in the latter z. B. to control the state of the cooking), in the monitoring and control of exhaust gases, in engine management systems or particulate filter systems in internal combustion engines (for example, in The latter for monitoring the regeneration) or in the Process engineering for the control or regulation of Processes (such as in automatic back roads, Bioreactors or incubators).

QUOTES INCLUDE IN THE DESCRIPTION

This list The documents listed by the applicant have been automated generated and is solely for better information recorded by the reader. The list is not part of the German Patent or utility model application. The DPMA takes over no liability for any errors or omissions.

Cited non-patent literature

• - Sensors and Actuators, 12 (1987) W. Wepner - SOLID-STATE ELECTROCHEMICAL GAS SENSORS *, pages 107-119 [0018]
• - Sensors and Actuators, 12 (1987) G. Hotzel and W. Wepner - POTENTIOMETRIC GAS SENSORS BASED ON FAST SOLID ELECTROLYTES *, pages 449-453 [0018]
• - Ionics, 3 (1997) O. Schaef, T. Widmer and U. Guth - In-Situ Formation of Thin Film Like β '' - Alumina Layers on Alumina Alumina Substrates, pages 277-281 [0067]
• M. Yamauchi, Y. Itagakia, H. Aono, and Y. Sadaoka, "Reactivity and stability of rare earth oxides - Li 2 CO 3 mixtures", J. Ot European Ceramic Society, 28 (2008), 27-34 [0069]

Claims (23)

Potentiometric sensor for the combined determination of the concentration of a first and a second gas constituent of a gas into a measuring gas space ( 1 ) can be introduced or present gas sample (P), comprising a cation-conducting, in particular a group-1-cation-conducting, in particular a Na ⁺ -ion conducting solid electrolyte ( 2 ), a first, with respect to the first and / or the second gas component gas-sensitive material having, in gas communication with the sample gas space and / or standing and with the solid electrolyte in two-dimensional contact measuring electrode ( 3 ), a second, with respect to the first and / or the second gas component gas-sensitive material having, in gas communication with the sample gas space and / or standing, with the solid electrolyte in two-dimensional contact and electrically isolated from the measuring electrode reference electrode ( 4 ), and one with the solid electrolyte ( 1 ) in surface contact, electrically isolated from the measuring electrode and from the reference electrode and gas-tight against the measuring gas chamber lockable and / or closed reference electrode ( 5 ). Potentiometric sensor according to the preceding claim, characterized in that the sensor is designed to determine at least one of, preferably all of the electrical potential differences listed below: a) the potential difference between the measuring electrode ( 3 ) and the reference electrode ( 4 ), b) the potential difference between the measuring electrode ( 3 ) and the reference electrode ( 5 ), c) the potential difference between the reference electrode ( 4 ) and the reference electrode ( 5 ). Potentiometric sensor according to the preceding claim, characterized in that the electrical (s) potential difference (s) a), b) and / or c) directly, preferably via a voltmeter, at the two respectively involved electrodes ( 3 . 4 . 5 ) can be tapped and / or that the sensor is designed so that the potential difference a) depends only on the partial pressure of the first gas component, but not the partial pressure of the second gas component, that the potential difference c) only from the partial pressure of the second gas component, but not depends on the partial pressure of the first gas component and / or that the potential difference b) depends on the partial pressures of the first and the second gas component. Potentiometric sensor according to one of the preceding claims, characterized in that the measuring electrode ( 3 ), the reference electrode ( 4 ) and / or the reference electrode ( 5 ) an electrically conductive electrode lead ( 3a . 4a . 5a ), with the aid of which the electrical potential of the measuring, reference and / or reference electrode can be tapped off, wherein preferably the electrode lead (s) ( 3a . 4a . 5a ) is formed at least in sections as a metal wire and / or as a printed layer is / are, and / or that the measuring electrode ( 3 ), the reference electrode ( 4 ) and / or the reference electrode ( 5 ) has an electrode grid, in particular an electrode grid in the form of a printed, partially transparent layer containing Au or Pt. Potentiometric sensor according to one of the preceding claims, characterized in that the first and / or the second gas component is one of the following gases: CO ₂ , SO ₂ , O ₂ , NO ₂ ; wherein preferably the first gas component CO ₂ and the second gas component O ₂ , or vice versa. Potentiometric sensor according to one of the preceding claims, characterized in that the reference electrode ( 5 ) by means of at least one gas-impermeable layer ( 6 ) gas-tight with respect to the sample gas space ( 1 ) is lockable and / or completed. Potentiometric sensor according to the preceding claim, characterized in that the at least one gas-impermeable layer ( 6 ) has at least one preferably sintered glass layer, in particular at least one printed layer and / or that the at least one gas-impermeable layer ( 6 ) is at least partially formed of the same material as the solid electrolyte ( 2 ). Potentiometric sensor according to one of the preceding claims, characterized in that the reference electrode ( 5 ) is a third, with respect to the first and / or the second gas component gas-sensitive material having and in solid contact with the solid electrolyte electrode and / or that the sensor from a measuring gas chamber ( 1 ) gas-tight separated reference gas space ( 7 ) and / or that of the sample gas space ( 1 ) a reference gas space ( 7 ) is separable, wherein the reference electrode ( 5 ) Can be brought in gas communication with the reference gas space and / or is. and / or that the reference electrode ( 5 ) is a third having only based on a solid-state reaction material and in contact with the solid electrolyte in contact electrode. Potentiometric sensor according to the preceding claim, characterized in that the measuring gas space ( 1 ) and the reference gas space ( 7 ) on two opposite sides of the solid electrolyte ( 2 ) are formed and / or are formed, wherein preferably the gas-tight separation of these two gas spaces ( 1 . 7 ) from each other by means of the solid electrolyte ( 2 ) and / or by means of a separate, preferably connected to the solid electrolyte gas seal ( 8th ), in particular a glass seal, can be formed and / or formed, and / or that the measuring electrode ( 3 ) and the reference electrode ( 4 ) are formed on a first surface (O1) of the solid electrolyte and adjacent to the latter and that the reference electrode (O1) 5 ) is formed on a second surface (O2) opposite the first surface of the solid electrolyte and adjacent to the solid electrolyte. Potentiometric sensor according to one of the preceding claims, characterized in that the solid electrolyte of a beta-aluminate compound cation beta-Al ₂ O ₃ with one of the cations Na ⁺ , K ⁺ , Li ⁺ , Mg ²⁺ , Ca ²⁺ , Ag ⁺ , Pb ²⁺ , Sr ²⁺ , Ba ²⁺ , in particular of a Na ₂ O.nAl ₂ O ₃ compound with n = 5,..., 11, and / or contains such a compound and / or that the solid electrolyte of M _{1 + x} Zr ₂ Si _x P _3-x O ₁₂ with 0 ≤ x ≤ 3 with a cation M from one of the cations Na ⁺ , K ⁺ , Li ⁺ , preferably from Na _{1 + x} Zr ₂ Si _x P _3-x O ₁₂ , and / or contains such a compound and / or that the solid electrolyte is at least partially formed as a printed layer on a substrate, in particular an Al ₂ O ₃ - or a ZrO ₂ substrate, as a shaped body produced from an initial powder by pressing and sintering of the starting powder is formed and / or as a cation beta formed via a solid-state reaction Al ₂ O ₃ diffusion layer is formed with one of the nine aforementioned cations in an Al ₂ O ₃ substrate. Potentiometric sensor according to one of the preceding claims, characterized in that the first material and / or the third material comprises a pure or a mixture of an alkali and / or alkaline earth metal carbonate, in particular of Na ₂ CO ₃ , with Au and / or Pt, wherein preferably a mixture of a rare earth metal oxide additive and / or a rare earth metal carbonate additive is added to the mixture. Potentiometric sensor according to one of the preceding claims, characterized in that the second material and / or the third material is a mixture of SiO ₂ and Na ₂ Si ₂ O ₅ with Au and / or Pt and / or a mixture of TiO ₂ and Na ₂ Ti ₃ O ₇ and / or Na ₂ Ti ₆ O ₁₃ with Au and / or Pt, and / or that the second material and / or the third material comprises a boron glass containing FeO and NiO. Potentiometric sensor according to one of the preceding claims, characterized in that the first material, the second material and / or the third material comprises a pure or a mixture of an alkali and / or alkaline earth nitrate, in particular of NaNO ₃ , with Au and / or Pt or a pure or a mixture of an alkali and / or alkaline earth sulfate, in particular of Na ₂ SO ₄ , with Au and / or Pt. Potentiometric sensor according to one of the preceding Claims, characterized in that the reference electrode, the measuring electrode and / or the reference electrode at least in sections a printed layer, in particular a screen printed and / or Inkjet printed layer, and / or at least in sections as one from a starting powder by pressing and sintering the starting powder formed molded body is formed. Potentiometric sensor according to one of the preceding claims, characterized by exactly one solid electrolyte ( 2 ). Potentiometric sensor according to one of the preceding claims, characterized in that on a surface of the solid electrolyte, preferably between the reference electrode and the measuring electrode, between the reference electrode and the reference electrode and / or between the measuring electrode and the reference electrode, at least one inert barrier ( 9 ), in particular a glass barrier, is arranged to cause short-circuit currents on this surface of the solid electrolyte. Potentiometric sensor according to one of the preceding Claims, characterized in that the electrical Isolation between the reference electrode, the measuring electrode and / or the reference electrode is realized in that the respective Electrode pairs spaced from each other on at least one surface of the solid electrolyte and adjacent to this surface are arranged. Potentiometric sensor according to one of the preceding Claims, characterized in that the reference electrode, the measuring electrode and / or the reference electrode as a planar layer with a layer thickness between 50 nm and 500 microns, preferably between 20 microns and 100 microns, is formed. Potentiometric sensor according to one of the preceding Claims, characterized in that the sensor a Heating element, wherein the heating element is preferably a resistance heating element in the form of at least one wound wire or at least one screen printed resistive element is and / or preferably one indirectly registered and / or enforceable heat flow includes, thereby preferably a temperature determination on the resistance heating element is feasible. Potentiometric sensor according to one of the preceding Claims, characterized in that the printed layer and / or the printed element by means of a mechanical printing process, in particular by means of a high, low, flat, through or instant printing method, is realized. Method for the combined determination of the concentration of a first and a second gas constituent of a gas in a measuring gas space ( 1 ) gas sample (P) by means of a potentiometric sensor, the sensor comprising: - a cation-conducting, in particular a group-1-cations-conducting, in particular a Na ⁺ -ion conducting, solid electrolyte ( 2 ), - a measuring electrode which has a gas-sensitive material with respect to the first and / or the second gas constituent, is in gas connection with the measuring gas space and is in surface contact with the solid electrolyte ( 3 ), - a second, with respect to the first and / or the second gas component gas-sensitive material having, in gas communication with the measuring gas chamber, in solid contact with the solid electrolyte in contact and electrically isolated from the measuring electrode reference electrode ( 4 ), and - one with the solid electrolyte ( 2 ) in surface contact, electrically isolated from the measuring electrode and from the reference electrode and sealed relative to the measuring gas chamber reference electrode ( 5 ), and wherein with the sensor at least one of, preferably all of the electrical potential differences listed below for determining the partial pressure and / or the concentration of the first and / or the second gas component in the gas sample are determined: - the potential difference between the measuring electrode ( 3 ) and the reference electrode ( 4 ), - the potential difference between the measuring electrode ( 3 ) and the reference electrode ( 5 ), - the potential difference between the reference electrode ( 4 ) and the reference electrode ( 5 ). Method for the combined determination of the concentration according to the preceding claim, characterized in that the Method with a sensor according to one of claims 1 to 20 is performed. Using a method or a sensor after one of the preceding claims for room air monitoring, in medical technology, in particular in the respiratory gas analysis, in the Consumer electronics, in particular in the field of protection technology or for oven controls, for control and / or monitoring of exhaust gases, in engine management systems of internal combustion engines, in particle filter systems and / or in process engineering for Control and / or regulation of processes such as in automated baking lines, bioreactors or incubators.

Images